Cost of Glycol/ton of cooling

[CanuckTJW]

Hello,

I am an industrial energy manager but not the strongest in refrigeration.

I've been recently asked to calculate the cost of glycol/ton of refrigeration.

Is this a normal industry metric?

Glycol is not really a consumable so I am not sure what this person is asking for. He has a previous figure of approximately $0.57/ton with Glycol at -2deg C. I think he wants to calculate the cost of operating the piece of equipment but using a straight forward Q=mCpdt should be sufficient to get the cooling load in BTU then calculate the cost in electricity from the compressor system.

I can see calculating the total cost of kWh using COP or kW/ton, operating hours, water and chemical consumption, pump operation etc

Any calculators or excel sheets out there I can vet and use?

Thanks in advance

[Segei]

Hi, Canuck TJW.

Probably, you were asked to estimate costs to produce 1 TR for glycol cooling. I'm doubt that you will be able to find any calculator or excel sheet to estimate this costs.
I will show you how estimation can be done.
Assume that refrigerant is ammonia. Plant with screw compressor(Frick RWB- II-134)and with evaporative condenser. Condenser water pump 20 HP. condenser fans 30 HP. Glycol pump 20 HP.
Glycol temperature is -2C or 28F. Assume that evaporation temperature is 15F or pressure is 28.4 psig. Assume that condensing pressure is 152 psig. For mentioned operating conditions this compressor will produce 261.7 TR and it will use 269 HP(engineering data from Frick). Total energy use 269 +20+20+30=339 HP or 339 x 0.75=254 kW
Energy use per 1 TR will be 254/261.7 = 0.97 kW/TR. At energy cost of 0.1$/kW costs to produce 1 TR will be around $0.1/TR.
In real life this number can vary from 0.08 to 0.12 because part load operation of compressor and condenser as well as different operating condensing pressures. Anyway it is much less that $0.57/TR that you mentioned.
Energy use and energy savings in industrial refrigeration is complicated issue, because we should evaluate energy use not one component but whole system. Personally, I think that all end users(people who have refrigeration plants) who want to invest in industrial refrigeration should have independent adviser. Otherwise a lot of money will be overspend.

[CanuckTJW]

Hi, Canuck TJW.

Probably, you were asked to estimate costs to produce 1 TR for glycol cooling. I'm doubt that you will be able to find any calculator or excel sheet to estimate this costs.
I will show you how estimation can be done.
Assume that refrigerant is ammonia. Plant with screw compressor(Frick RWB- II-134)and with evaporative condenser. Condenser water pump 20 HP. condenser fans 30 HP. Glycol pump 20 HP.
Glycol temperature is -2C or 28F. Assume that evaporation temperature is 15F or pressure is 28.4 psig. Assume that condensing pressure is 152 psig. For mentioned operating conditions this compressor will produce 261.7 TR and it will use 269 HP(engineering data from Frick). Total energy use 269 +20+20+30=339 HP or 339 x 0.75=254 kW
Energy use per 1 TR will be 254/261.7 = 0.97 kW/TR. At energy cost of 0.1$/kW costs to produce 1 TR will be around $0.1/TR.
In real life this number can vary from 0.08 to 0.12 because part load operation of compressor and condenser as well as different operating condensing pressures. Anyway it is much less that $0.57/TR that you mentioned.
Energy use and energy savings in industrial refrigeration is complicated issue, because we should evaluate energy use not one component but whole system. Personally, I think that all end users(people who have refrigeration plants) who want to invest in industrial refrigeration should have independent adviser. Otherwise a lot of money will be overspend.

Segei

Thanks for the direction butI have couple of questions of clarity.

I am not sure why you are using a energy costs (kWh) of $0.1/kW? Did you mean $0.1/kWh? If so that is fine however your load calculation does not included any hours of operation unless you are assuming this is $0.1/ton/hr. If not then demand cost (kW) are much more than $0.1/kW but closer to $4.5/kW

Thanks

[Segei]

Segei

Thanks for the direction butI have couple of questions of clarity.

I am not sure why you are using a energy costs (kWh) of $0.1/kW? Did you mean $0.1/kWh? If so that is fine however your load calculation does not included any hours of operation unless you are assuming this is $0.1/ton/hr. If not then demand cost (kW) are much more than $0.1/kW but closer to $4.5/kW

Thanks

Canuck TJW

Kw is unit of power. energy = power x time kWh is unit of energy. Mentioned refrigeration plant has power 339 HP or 254 kW. If we run this plant for 1 hour, it will use 254 kWh of energy and will produce 261.7 TR. I meant that cost of energy is $0.1/kWh. Hours of operation included when we multiply power by time. Cost to produce 1 TR is $0.1/TR. If we need to produce 1000 TR, we should run 100 TR plant for 10 hours or 1000 TR plant for 1 hour. Demand(kW) is power not energy.
What for do you need the cost of producing 1 TR?

[sandybapat]

Segei

What you have indicated is power consumption at shaft of compressor. We have to account for drive losses and motor efficiency also. For example if I assume 2% transmission losses and motor efficiency of 95% then for compressor shaft power of 269 HP (201 kW), I would calculate power input to compressor motor as under:
201x1.02/0.95 = 215.8 kW. Similarly for all utility motors. I have to consider the power required for the oil pump if installed. I know Frick RWBII-134 does not need oil pump.

[CanuckTJW]

Canuck TJW

Kw is unit of power. energy = power x time kWh is unit of energy. Mentioned refrigeration plant has power 339 HP or 254 kW. If we run this plant for 1 hour, it will use 254 kWh of energy and will produce 261.7 TR. I meant that cost of energy is $0.1/kWh. Hours of operation included when we multiply power by time. Cost to produce 1 TR is $0.1/TR. If we need to produce 1000 TR, we should run 100 TR plant for 10 hours or 1000 TR plant for 1 hour. Demand(kW) is power not energy.
What for do you need the cost of producing 1 TR?

Thank Segei. I think I demonstrated I understand the difference between demand (kW) and electrical energy (kWh). I was just asking for you to clarify your assumption and notation is all; you did and I thank you. I naturally assumed it was a typo more than anything else

I also agree with Sandy that efficiencies should be considered but also would add the loading of the different motors. Motors are very often not 100% loaded as well.

Overall, this is the approach I was planning on taking as stated in my OP. I just wanted to check if what I was being asked was something I hadn't heard of before. As you pointed out what the person was asking for and what they wanted were different. It is one of the many hurdles I find in energy management. That is, people have an idea of what they want but not sure how to ask for it or go about it. It is then part of my job to decipher that and provide the metrics they need to make decisions.

Thanks again

[Segei]

Segei

What you have indicated is power consumption at shaft of compressor. We have to account for drive losses and motor efficiency also. For example if I assume 2% transmission losses and motor efficiency of 95% then for compressor shaft power of 269 HP (201 kW), I would calculate power input to compressor motor as under:
201x1.02/0.95 = 215.8 kW. Similarly for all utility motors. I have to consider the power required for the oil pump if installed. I know Frick RWBII-134 does not need oil pump.

Sandybapat

This is just example. As I mentioned before, in real life number $0.1/TR can go up or down by 20-25%. Major factors are condensing pressure and compressor load.
I agree about el. motor efficiency and oil pump. You mentioned 2% transmission losses. I agree as well if losses in the electrical grid, but not in compressor room.

[Segei]

Thank Segei. I think I demonstrated I understand the difference between demand (kW) and electrical energy (kWh). I was just asking for you to clarify your assumption and notation is all; you did and I thank you. I naturally assumed it was a typo more than anything else

I also agree with Sandy that efficiencies should be considered but also would add the loading of the different motors. Motors are very often not 100% loaded as well.

Overall, this is the approach I was planning on taking as stated in my OP. I just wanted to check if what I was being asked was something I hadn't heard of before. As you pointed out what the person was asking for and what they wanted were different. It is one of the many hurdles I find in energy management. That is, people have an idea of what they want but not sure how to ask for it or go about it. It is then part of my job to decipher that and provide the metrics they need to make decisions.

Thanks again

Canuck TJW.

No doubt that you know difference between power and energy. You mentioned about demand and I decided to give more info than it required. However, it is everything clear now.
I'm focused on energy savings in industrial refrigeration for 15 years and I understand that it is not easy for you to make the right decision related to energy savings in our industry. There are several pitfalls and I'd like to share with you.
1. Snake oil. People try to sell you some "magic" devices or additives which will save a lot of energy. We discussed several devices on this forum. Usually, I ask these people to show calculation of energy savings. Any energy savings can be calculated. They will give references of second, third....tens party but no calculation, because calculation can be checked.
2. Inflated energy savings. Probably, that is happen to you. Someone gave you inflated(5 times) energy use and energy savings will be inflated as well.
3. Expensive and cost effective approaches. Repiping, new equipment, VFDs, PLCs save energy but they are very expensive and have long payback. Optimization of the refrigeration plant operation is the most cost effective way to save energy in our industry. Typical payback is 2-3 months. Recently, visited plant where contractor proposed to save 1,000,000 kWh but required $1,000,000 investments. Found that the same 1,000,000 kWh can be saved by investing $50,000.
Hopefully, this info will be useful for you.

[sandybapat]

Segei

Incidentally, I remember a situation. For a refrigeration plant for catering pharmaceutical company the head for the secondary brine pump was suggested by the consultant was 60 m and the pump was selected accordingly. Obviously the speed of the pump selected was 2950 rpm. On actual installation it was found that the actual head required is just 30 m. Consultant suggested to use VFD for this pump. Since the load was constant, it was suggested that the drive motor can be replaced with smaller rating and having 1450 rpm speed. That worked and a much cheaper solution was found.

[hookster]

Is it not a question rather of comparison on cost/TR between direct and secondary cooling?
The capital cost for glycol equipment will be greater due to larger heat exchangers, pumps, pipework, massflow rate etc.
This all results in secondary systems having a higher operating cost/TR